1. Field of The Invention
This invention relates to an apparatus and method for manufacturing wood pulp.
More particularly, this invention relates to an apparatus and method for manufacturing ground wood pulp from ligno-cellulose containing material of the type including a grinding disc mounted rotatably about a shaft centered on both end surfaces of the grinding disc, retaining means adapted to retain and force at least one piece of material to be ground against at least one of the end surfaces and means to supply an aqueous liquid to the material. Each piece of material is forced against the grinding end surface with the fibers of the material substantially directed in the plane of the grinding surface.
2. Description of The Prior Art
The conventional method for defibrating ligno-cellulose containing material by grinding involves pressing the material, normally pieces of timber, against the cylindrical surface of a grindstone or grinding disc. This method, invented over a hundred years ago, still dominates the production of mechanical paper pulp.
Despite many attempts at improvements of this method during the course of years, the method nevertheless has its decided limitations. Currently, the grinding surface often consists of sharp grains of ceramic material embedded in a binder. When the grains are worn, the surface of the disc becomes smooth and production is reduced while at the same time the pulp is ground too finely. The grinding disc must then be redressed by means of a sharpening roller, which pulls up a new grinding pattern. Because this results in the working surface of the disc being changed continuously during the grinding operation, the quality of the pulp cannot be maintained uniform. In addition, the grinding grains tear off many of the fibers resulting in a short-fiber pulp with poor strength qualities. By the redressing operation grains are torn from the disc and admixed to the pulp, causing wear in the machines used for subsequent treatment of the pulp.
Although attempts have been made to replace the cylindrical working surface by a patterned steel surface, no practical success has been obtained for several reasons, among which the difficulty of producing a curved steel grinding surface at a reasonable cost is the most prominent one.
More recently, it has been proposed to provide the cylindrical grinding surface with ribs, bars or similar straight projections made of steel, a metal alloy or other suitable hard material, the upper cutting portions of which have been made arcuate with certain definite heights and arc radii. While a good final product is obtained by the use of such a grinding disc, the grinding means are very expensive. Thus, such discs must be made with very fine tolerances and must be replaced rather often because they wear like all grinding appliances and in this way relatively soon completely lose their good grinding qualities.
A feature common to all the grinding discs of the kinds referred to hereinbefore is that the removal capacity of the disc can only be increased to a certain limit which is determined by the length of the grinding disc, its diameter and its rotational speed. This limit has already been reached a long time ago in modern grinding plants. When attempting to further increase the diameter of the grinding disc or its length, the bending moment acting on the horizontal shaft carrying the disc will exceed tolerable limits, and the distance between the bearings will be increased to a corresponding practically intolerable extent. At high rotational speed the stresses on the grinding disc will be large and the grinding rendered more difficult by the wood material being thrown out from the grinding surface by centrifugal force, a serious disadvantage.
Modern grinders with automatic replenishment of material such as blocks, short logs or lumps of timber, in contrast to chips which are treated in disc refiners, usually have two compartments for the pieces of wood and thus two subsequent grinding areas on the cylindrical surface of the grinding disc. The compartments are filled batch-wise. When one compartment is being filled, the effective grinding surface is reduced to one half, which causes a corresponding reduction in the load on the driving electrical motor and thereby disturbances in the electrical power supply. Moreover, the grinding disc is loaded unevenly and the shaft bearings are therefore exposed to unduly large stresses. Since the disc rotates about a horizontal axis and since the cylindrical surface forms the grinding surface, it is obvious that it is not possible to distribute a greater number of compartments over the grinding surface.
All the aforementioned difficulties together have caused complete stagnation in the further development of wood pulp grinding machines of the general type described, and the abovestated disadvantages have been accepted in the art as unavoidable.